Short story pre-1990 involving intelligent rats

Question

I just spotted this question which reminded me of a short story with some similarities in English that I read in the late 1980s (it was reprinted in a primary school textbook as part of a comprehension exercise so I have no idea about the cover, I'm afraid). It definitely isn't the rats of NIMH.

It was written in the form of a series of letters written by a scientist documenting the rapid increase in intelligence of an individual rat or small group of rats on an island. I think he was experimenting on it/them in some way, but I can't be sure about that. The tone of his letters grows increasingly alarmed. The final letter, written in suspiciously broken English (possibly also described as in terrible handwriting), then says (paraphrasing) "all the rats have suddenly died out and I am about to kill myself, nothing to see here, probably best if you do not visit this island again".

Does this ring any bells with anyone?

Answer 1

I'm wondering if it is the short story Barney by Will Stanton? It's from the 50s so could have made its way into a collection of short stories at a later date and reads like a chronological journal and the scientist logging it finds Barney to be smarter and smarter than he thought. The last entry is indeed the narrator apologising for his bad handwriting:

I sprayned my wrist is why this is written so bad.

and the narrator says:

Do not look for my body as I will caste myself into the see.

The only difference here is that the story only revolves around one rat on an island.

Answer 2

Could it be Flowers for Algernon?

The story is told through a series of journal entries written by the story's protagonist, Charlie Gordon, a man with a low IQ of 68 who works a menial job as a janitor in Donnegan's Plastic Box Company. He is selected to undergo an experimental surgical technique to increase his intelligence. The technique had already been successfully tested on Algernon, a laboratory mouse. The surgery on Charlie is also a success, and his IQ triples. He realizes his co-workers at the factory, who he thought were his friends, only liked him around so they could tease him. His new intelligence scares his co-workers, and they start a petition to have him fired, but when Charlie learns about the petition, he quits.

But:

As Charlie's intelligence peaks, Algernon's suddenly declines—he loses his increased intelligence and mental age, and dies afterward, buried in the back yard of Charlie's home. Charlie realizes his intelligence increase is also temporary. He starts to experiment to find the cause of the flaw in the experiment, which he calls the "Algernon–Gordon Effect". When he finishes his experiments, his intelligence regresses to its original state. Charlie is aware of, and pained by, what is happening to him as he loses his knowledge and his ability to read and write. He tries to earn back his old job as a janitor, and tries to revert to normal, but he cannot stand the pity from his co-workers, landlady, and Ms. Kinnian. Charlie states he plans to "go away" from New York and move to a new place. His last wish is for someone to put flowers on Algernon's grave.

Answer 3

The story is by Will Stanton and was published first in "Fantasy and Science Fiction" in 1951 and last in "Crime Fiction" by Ernst Klett Sprachen GmbH, a German educational publisher with an emphasis on the teaching of languages, as well a few times in between those two dates.